Electrodepositing aluminum



United States Patent ELECTRODEPOSITING ALUMINUM Mike A. Miller and Cole D. Baker, New Kensington, ltu,

assignors to Aluminum Company of Amer1ca, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application May 23, 1955, Serial N0. 510,571

8 Claims. (Cl. 204-14) This invention relates to electrodepositing aluminum, particularly from organo-aluminum electrolyes. Such electrolytes may be employed for electroplating or electroforming aluminum on any suitable cathode, and the invention is particularly suited to the production of bright, smooth, coherent and adherent electroplates on articles made of either aluminum or some other metal such as iron, steel, copper, brass, bronze, nickel, and the like. As generally used herein, the word aluminum includes aluminum base alloys.

It is generally recognized that aluminum cannot be successfully electrodeposited from an aqueous solution. However, both inorganic aluminum salt baths and organo-aluminum baths have heretofore been used as electrolytes for electrodepositing aluminum. Inorganic aluminum salt baths must be fused and operated at elevated temperatures, with consequent fuming and composition control problems, and they produce rather thin electrodeposits. Organo-aluminum baths heretofore proposed usually are somewhat unstable and some contain flammable or even potentially explosive organic components, aside from being relatively expensive to make up.

.A general object of the invention is to provide improved, less expensive organo-aluminum electrolytes, and processes utilizing then, for electrodepositing aluminum. Particular objects include providing for the use of moderate operating temperatures, with stable, less flammable electrolytes, providing for high rates of electrodeposition, and producing either thin or thick aluminum electroplates of smooth, coherent and adherent character, on either aluminum or other metals.

We have discovered that very useful organo-aluminum electrolytes for electrodepositing aluminum may be made up with guanidine hydrochloride as a component. The electrolytes referred to consist essentially of (a) at least one aluminum halide from the group consisting of aluminum chloride and aluminum bromide, (12) guanidine hydrochloride, and (c) at least one aromatic hydrocarbon from the group consisting of benzene, toluene and xylene. Further, we have found that such electrolytes should have a mol ratio of aluminum halide from the group mentioned to guanidine hydrochloride which lies between 1.7:1 and 2.9:1, preferably about 2:1, and a mol ratio of aromatic hydrocarbon from the group mentioned to guanidine hydrochloride of at least about 0.4:1, preferably at least about 4:1. The preferred aluminum halide is aluminum chloride, although aluminum bromide or mixtures of aluminum chloride and aluminum bromide may be employed to provide the necessary aluminum halide content. The preferred aromatic hydrocarbon is toluene, although either of the others or mixtures may be employed to provide the necessary aromatic hydrocarbon content.

We have found it both convenient and preferable, when employing an aromatic hydrocarbon from the group mentioned, to employ an amount which is at least suflficient to saturate the electrolyte. A mol ratio of benzene ice to guanidine hydrochloride on the order of 6:1 or 7:1 will saturate the electrolyte; with toluene, a mol ratio on the order of 5:1 or 6:1 will produce saturation; and with xylene, a mol ratio on the order of 4:1 or 5:1 will produce saturation. The aromatic hydrocarbon may, of course, be added in an amount well in excess of that required to produce saturation, as the excess will tend to float above the saturated mixture. Even when the electrolyte is stirred or otherwise agitated during electrodeposition, as is usually desirable, we have found that some excess aromatic hydrocarbon, if present, may be dispersed throughout the electrolyte without detrimental effect.

In some cases it may be desirable to provide a smoothing or brightening agent in the electrolyte, especially when relatively thick electrodeposits are to be made. For this purpose we have found a small addition of naphthalene beneficial. For example, a mol ratio of naphthalene to guanidine hydrohalide of at least 0.1:1, preferably 0.3:1 may be employed, and larger amounts may be employed, if desired. Various other materials are compatible with the electrolyte and may be added for similar or other purposes.

Since moisture is undesired in the electrolyte, an

argon or other non-oxidizing gas atmosphere may be maintained above it. Further, we have found that the electrolyte may be treated with lithium hydride, conveniently by having at least 0.01, preferably about 0.02 or more, mol of lithium hydride per mol of guanidine hydrohalicle present therein, to shorten the usually required break in period and to overcome the adverse effects of anticipated contamination of the electrolyte with moisture. The lithium hydride appears to react in the electrolyte only to the extent that moisture enters the electrolyte, and its presence in even a minute quantity has been found beneficial over extended periods of time.

An organo-aluminum electrolyte of the character described may be used for electrodepositing aluminum by employing an aluminum anode and making the article which is to receive the electrodeposit cathode in such electrolyte while maintaining the electrolyte fluid. We have found it both desirable and practical to maintain the electrolyte at a temperature below C., in order to minimize loss of volatile components and solution of both the aluminum anode and the aluminum. electrodeposit on the cathode. We have successfully operated such electrolytes well below room temperature, down to about 0 C., but prefer to operate at temperatures between 20 and 60 (3., usually about 30 C. We also prefer to stir or agitate the electrolyte during electrodeposition, by mechanical stirring or by vibration of either electrode, for example, especially when employing high rates of electrodeposition at current densities on the order of 20 to 60 amperes per square foot of cathode area.

A desirable alternative to merely causing. a direct current to How between anode and cathode is to employ a direct current upon which is superimposed an alternating current, conveniently a 60 cycle current of about 1 /2 times the magnitude of the direct current employed. It is also advantageous, especially when smooth, coherent electroplates of substantial thickness are to be produced,

to periodically reverse the direct current for a short time (whether or not a superimposed alternating current is employed), using a reversed direct current about 1 /2 to 4 times the direct current used for electrodeposition. We have found it convenient to reverse the direct current approximately once a minute for about 1 /2 to 6 seconds, i. e. about 2 /2 to 10 per cent of the time.

In order to produce an adherent electroplate on an aluminum article, it has heretofore generally been necessary to employ a separate surface conditioning pretreatment. However, with an electrolyte of the character described, we have found that the aluminum article may advantageously be given only a conventional cleaning treatment and then be preliminarily made anode in the electrolyte at a current density of about to amperes per square foot, for about 3 to 10 minutes, before making it cathode for electrodeposition. This procedure permits of producing very satisfactory adherent electroplates.

The following is an example of our preferred electrolyte and preferred operating procedure. First, 84 grams of aluminum chloride and 30 grams of guanidine hydrochloride were mixed, fused, and cooled to about room temperature. Then, 219 milliliters of toluene (190 grams), 5 grams of naphthalene, and 0.05 grams of lithium hydride were added. This produced an electrolyte in which the mol proportions were approximately 2.0 mol aluminum chloride:1.0 mol guanidine hydrochloride:6.6 mol toluene (of which a little over 5 mol was soluble):0.1 mol naphthalene, with about 0.02 mol lithium hydride present therein. This electrolyte was then employed at about 30 C., with an argon atmosphere maintained above it, and with mechanical stirring. It was used for electrodeposition, employing an aluminum anode. Sheet specimens of 1100 aluminum alloy, die cast 380 aluminum alloy articles, and sheet specimens of mild steel were employed as cathodes and thus electroplated. The aluminum alloys were pre-treated as anode for 10 minutes, at a current density of 6 to 8 amperes per square foot. The plating current was applied for about 1 to 16 hours with the various materials as cathode, at current densities up to 20 amperes per square foot. Direct current, with superimposed 60 cycle alternating current of about 1 /2 times the magnitude of the direct current, was employed, with periodic reversal of the direct current about once a minute for about 6 seconds, at double the plating current density. Electroplates of up to about 0.001 inches in thickness per hour of electrodeposition were readily produced.

Additional electrolytes illustrative of those useful in 3. An electrolyte in accordance with claim 1, also having lithium hydride present therein.

4. An electrolyte for electrodepositing aluminum consisting essentially of at least one aluminum halide from the group aluminum chloride and aluminum bromide, guanidine hydrochloride, and at least one aromatic hydrocarbon from the group benzene, toluene and xylene, the mol ratio of aluminum halide from the group mentioned to guanidine hydrochloride being about 2: 1, and the mol ratio of aromatic hydrocarbon from the group mentioned to guanidine hydrochloride being at least about 4: 1.

5. An electrolyte for electrodepositing aluminum consisting essentially of aluminum chloride, guanidine hydrochloride, and toluene, the mol ratio of aluminum chloride to guanidine hydrochloride being about 2:1, and the amount of toluene being at least sufficient to saturate the electrolyte.

6. A process of electrodepositing aluminum comprising employing an aluminum anode and making an article which is to receive the electrodeposit cathode in an electrolyte consisting essentially of at least one aluminum halide from the group aluminum chloride and aluminum bromide, guanidine hydrochloride, and at least one aromatic hydrocarbon from the group benzene, toluene and xylene, the mol ratio of aluminum halide from the group mentioned to guanidine hydrochloride lying between 1.7:1 and 2.9:1, and the mol ratio of aromatic hydrocarbon from the group mentioned to guanidine hydrochloride being at least about 0.4:1, while maintaining the electrolyte fluid at a temperature below 80 C.

7. A process in accordance with claim 6, in which the article to receive the electrodeposit is an aluminum article and it is preliminarily made anode in the electrolyte at a current density of at least about 5 amperes per square foot, for at least about 3 minutes, before electrodeposition is begun.

8. A process of electrodepositing aluminum comprising employing an aluminum anode and making a metallic article cathode in an electrolyte consisting essentially of at least one aluminum halide from the group aluminum accordance with the invention include the following in mol proportions: chloride and aluminum bromlde, guanidine hydrochloride,

A1011 AIBIa OH5N3.HC1 CflHQ CsHszCHa C H4(CH3)2 CIUHH 11111 and at least one aromatic hydrocarbon from the group benzene, toluene and xylene, the mol ratio of aluminum halide from the group mentioned to guanidine hydrochloride being about 2:1, and the mol ratio of aromatic hydrocarbon from the group mentioned to guanidine hydrochloride being at least about 4: 1, while maintaining the electrolyte fluid at a temperature between 20 and 60 C.

References Cited in the file of this patent UNITED STATES PATENTS Brenner et a1. Sept. 8, 1953 Safranek Oct. 26, 1954 

1. AN ELECTROLYTE FOR ELECTRODEPOSITING ALUMINUM CONSISTING ESSENTIALLY OF AT LEAST ONE ALUMINUM HALIDE FROM THE GROUP ALUMINUM CHLORIDE AND ALUMINUM BROMIDE, GUANIDINE HYDROCHLORIDE, AND AT LEAST ONE AROMATIC HYDROCARBON FROM THE GROUP BENZENE, TOLUENE AND XYLENE, THE MOL RATIO OF ALUMINUM HALIDE FROM THE GROUP MENTIONED TO GUANIDINE HYDROCHLORIDE LYING BETWEEN 1.7:1 AND 2.9:1 AND THE MOL RATIO OF AROMATIC HYDROCARBON FROM THE GROUP MENTIONED TO GUANIDINE HYDROCHLORIDE BEING AT LEAST ABOUT 0.4:1. 